An NBC suit won't guarantee keeping it out unfortunately, at least not a military one...
A properly applied hazmat suit with a sealed air supply, yes, that'll work. But then you'll need a decon unit to remove it while maintaining ~100% protection.
But anything has been demonstrated to be more effective than nothing, especially in conjunction with other measures - any face covering plus: look with your eyes not your hands, clean/sanitised hands, don't put your hands on your face, don't kick random strangers, etc. It all helps (more or less).
And in a surgical setting, the mask is to protect the patient, a visor is to protect the wearer (from splashes etc having direct contact with open mucous membranes).